Illumination system for a printing press

ABSTRACT

An illumination system for a web travelling from upstream to downstream in a longitudinal direction in a printing press is described. The illumination system includes a first and a second illuminator for emitting light, each illuminator having a long axis arranged in a lateral direction. Further, the system includes a first and a second reflector, with the second reflector being arranged downstream from the first reflector. Each reflector has a surface for reflecting light from a corresponding illuminator toward the web, wherein a cross-section of each reflecting surface is a portion of a parabola having a focal point, or a portion of an ellipse, having two focal points. An illuminator is located at a corresponding focal point.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/385,311, filed Mar. 10, 2003.

FIELD OF THE INVENTION

The present invention relates generally to an illumination system for aprinted work on a web in a printing press, and more particularly, toreflectors for reflecting light from an illuminator to the printed workon the web.

BACKGROUND OF THE INVENTION

A web-offset printing press includes an inking assembly for each colorof ink used in the printing process. Each inking assembly includes anink reservoir and a plurality of hard nylon keys or a segmented bladedisposed along the outer surface of an ink fountain roller. The amountof ink supplied to a roller train of the press and ultimately to asubstrate, such as a web of paper, is adjusted by changing the spacingbetween the edge of the blade segments or the nylon keys and the outersurface of the ink fountain roller. The position of each blade segmentor each key relative to the ink fountain roller is independentlyadjustable via an ink control system to thereby control the amount ofink fed to a corresponding longitudinal strip or ink key zone of thesubstrate.

Typically, ink is spread laterally from one longitudinal zone toadjacent zones due to the movement of vibrator rollers, which oscillatein a lateral direction relative to the substrate. The amount of ink onthe ink fountain roller itself is also adjustable by changing the anglethrough which the ink fountain roller rotates each stroke. Thisgenerally occurs by adjusting a ratchet assembly, as is known in theart.

While such a printing press is running, a camera is typically used tocontinually monitor the printed output and to make appropriate ink keyadjustments in order to achieve appropriate quality control of the colorof the printed image. Specifically, the camera moves across the web tocollect images of color patches on the moving web. Each pixel of thecolor patch images is then processed, and assigned a color value. Eachcolor value is compared against a desired color value. If the absolutedifference between the desired color value and the determined colorvalue is outside some predetermined tolerance, the ink key is thencontrollably adjusted, thereby effecting a change in the ink flow rate.

It is not uncommon for printed images on the web, color patches inparticular, to be corrupted by some printing artifact such as the effectof a paper fiber on the blanket roller (commonly known as a hickey), adroplet of ink, an indentation on the blanket, a slime hole in thepaper, a scratch on the plate, or some other such defect. In this case,the measured color values of a defective color patch may not accuratelyreflect the color within the printed work itself. While methods fordetecting a small defect in a color patch exist in marked color controlsystems, they are generally limited to eliminating small defects that donot encompass a relatively large portion of the color patch.Furthermore, these color control systems use techniques that assume thatthe color properties of the printed work remain constant over a definedarea. However, the color properties of the print work may not remainconstant. As a result, other techniques are needed to detect defects.

Color control systems for printing presses not requiring the use ofcolor patches, or markless color control systems have been developed.Such markless color control systems measure color values in the printedwork itself. Since the color of the printed work is measured directly inthe markless systems, the correspondence between color patches and thework is not in question. However, these systems do not detect defects onthe printed work. Even though the marked color control systems areconfigured to detect defects in the printed work, these defect detectiontechniques are applied to marked color control systems only.

For example, printing presses typically include a defect detectionsystem as are known in the art. This type of defect detection systemscans, and acquires an image of the printed web. The acquired image issubsequently compared to a stored digital template image. Anydiscrepancy between the acquired image and the template image beyondsome tolerance is considered to be a defect. The isolated defects arethen logged in a data file. When the systems detect a large change incolor due to a change in inking level, a non-isolated defect is reportedover a large portion of the web. When non-isolated defects are reported,an alarm will subsequently be set off to alert an operator to takeappropriate corrective action.

Once a printed product is determined to be acceptable, the defectdetection control systems will typically establish a new template imageby scanning the acceptable printed product. The defect detection controlsystem is not fully functional until a printed product is determinedacceptable. While a template image can be collected before the printedproduct is considered acceptable, the template image may actuallycontain a defect, and an actual defective image may be consideredacceptable or good, and therefore no corrective action is taken.

Furthermore, the printed product may have subtle defects even when it isjudged acceptable. For example, a printing plate may have been scratchedbefore the printing process started, or a blanket flaw such as a hickeyor indentation may have been present.

The makeready process typically includes a visual comparison andinspection of a print product against a contract proof. This visualcomparison and inspection process establishes that no formatting errorsare introduced into the press between making the contract proof andputting the printing plates on press. However, typical defect detectioncontrol systems do not allow for a template image that has beencollected based on a contract proof, or based on a digitalrepresentation of the printed work that was used to create the printingplate.

Traditionally, color control systems and defect detection controlsystems are two separate systems operating on a printing press. Theseseparate systems utilize separate web scanning mechanisms. Imageprocessing is often duplicated in these two control systems as well.

SUMMARY OF THE INVENTION

The invention provides an illumination system for a web travelling fromupstream to downstream in a longitudinal direction in a printing press.A lateral direction is defined to be substantially perpendicular to thelongitudinal direction. The system includes a first and a secondilluminator for emitting light, each illuminator having a long axisarranged in the lateral direction. Further, the system includes a firstand a second reflector, with the second reflector being arrangeddownstream from the first reflector. Each reflector has a surface forreflecting light from a corresponding illuminator toward the web,wherein a cross-section of each reflecting surface is a portion of aparabola having a focal point. A corresponding illuminator is located ateach focal point.

In another aspect, the invention provides an illumination system for aweb travelling from upstream to downstream in a longitudinal directionin a printing press. A lateral direction is defined to be substantiallyperpendicular to the longitudinal direction. The illumination systemincludes a first and a second illuminator for emitting light, eachilluminator having a long axis arranged in the lateral direction. Thesystem also includes a first and a second reflector, the secondreflector arranged downstream from the first reflector. Each reflectorhas a surface for reflecting light from a corresponding illuminatortoward the web, wherein a cross-section of each reflecting surface is aportion of an ellipse having a first and a second focus. The firstilluminator is located at the first focus of the first reflector, andthe second illuminator is located at the first focus of the secondreflector. The second focus of the first reflector and the second focusof the second reflector are substantially overlapping.

Other features and advantages of the invention will become apparent byconsideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of a printing press;

FIG. 2 is a side view of a scanner assembly;

FIG. 3 is a perspective view of a lighting element of the scannerassembly;

FIG. 4 is a sectional view of the lighting element with a slit aperture;

FIG. 5 is a sectional view of an alternative embodiment of the lightingelement;

FIG. 6 is a perspective view of the lighting element emitting light froma single point;

FIG. 7 is a perspective view of an image sensor arrangement;

FIG. 8 is a flow chart of a control system;

FIG. 9 is a table indicating input and output rules;

FIG. 10 is a perspective view of a portion of printing press includingan alternative embodiment of the control system;

FIG. 11 is a side view of one embodiment of an arrangement of twoparabolic reflectors;

FIG. 12 is a side view of another embodiment of an arrangement of twoparabolic reflectors;

FIG. 13 is an illustration of a group of parabolic curves;

FIG. 14 illustrates various utilization angles for different parabolicreflectors;

FIG. 15 plots utilization angles versus focal length for variousclearance lengths;

FIG. 16 is a graph of angle versus distance;

FIG. 17 is a graph of light intensity versus distance for an illuminatorand parabolic reflector;

FIG. 18 is a side view of another embodiment of an arrangement of twoparabolic reflectors;

FIG. 19 is a graph of light intensity versus distance for thearrangement illustrated in FIG. 18;

FIG. 20 is a side view of a compound reflector including a circularportion and a parabolic portion; and

FIG. 21 is a perspective view of the reflector illustrated in FIG. 20.

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A control system 130 according to the present invention is shown inFIG. 1. The control system 130 includes a single scanner assembly 134for both color control and defect detection purposes, and a singlesystem processor 138. The scanner assembly 134 collects image data froma web 142 moving in a direction 143. Once collected, the acquired imagedata is transferred to the processor 138 for processing in a colorcontrol subsystem and a defect detection subsystem. Such processingincludes color control, such as ink level adjustment, and defectdetection. The ink level adjustment information is then communicated tothe associated printing press to effect a change in ink level whendeemed necessary as is known in the art.

Generally, the scanner assembly 134 includes an illumination systemwhich illuminates the moving web 142, an image sensor which sensesreflected light from the moving web 142, and any associated opticelements required to appropriately disperse the illumination or directlight to the image sensor. Referring now to FIG. 2, a preferred scannerassembly 134 is shown. The scanner assembly 134 includes a pair of lightsources or lighting elements 144 located upstream and downstream from animage sensor 145. Each lighting element 144 further includes anilluminator 146, arranged substantially parallel to the moving web 142and substantially perpendicular to the direction 143, and a reflector150.

The illuminator 146 provides illumination to the web 142 with a pair offluorescent bulbs, for example. As the web 142 moves, an encoder signalfrom the printing press drives a shutter mechanism to triggeracquisitions of data. At each acquisition, the image sensor 145 senses aportion of the efflux light that is reflected from the web 142.

When high-speed web or fine resolution printing is desired, theilluminator 146 is typically powered by a high frequency power supply tomaintain a relatively constant strength of illumination from one imageline to the next. In the preferred embodiment, the illuminator 146 is atube-shaped halogen bulb with a filament running parallel to the web142. The tube-shaped halogen bulb typically provides illuminationstability until its point of failure, and the filament providessubstantially uniform illumination across the web 142. Otherillumination devices such as a series of conventional incandescent bulbsmay also be used.

Referring now to FIGS. 2-3, the reflector 150 is shown which is utilizedto make efficient use of light. The reflector 150 extends substantiallyparallel to the illuminator 146. In the preferred embodiment, thereflector 150 has a general shape of a part of an ellipse 154, which hastwo foci 158, 162. The illuminator 146 is substantially aligned at thefirst focus 158. The second focus 162 is generally at a point on or justabove the web 142 and below the image sensor 145. The two reflectors 150are aligned such that the second focus 162 of each reflector 150 issubstantially coincident.

FIG. 4 shows another embodiment of the lighting element 144. Theilluminator 146 as shown in FIG. 4 is positioned such that a 45° angleis made between the web 142 and a line 166 connecting the two foci 158,162. A slit aperture 170 is placed near the focus 162 to obstruct thelight that impinges the web 142 at an angle substantially different from45°. The reflector 150 is designed to utilize only the reflected lightthat passes through the aperture 170. The reflector 150 includes a blindspot 174. The light reflected from the blind spot 174 generally does notpass through the aperture 170. The blind spot 174 is preferably given aflat black finish to absorb a significant portion of the light from theilluminator 146. If the reflector 150 is left reflective at the blindspot 174, the light that leaves the illuminator 146 toward the blindspot will be reflected back through an illuminator surface. Since thereflected light does not re-enter perpendicular to the illuminatorsurface, the illuminator surface subsequently refracts and scatters thereflected light. Thus, the blind spot 174 is preferably darkened.

The lighting elements 144 are preferably packaged in an enclosure suchthat all the light emitting from the enclosure leaves through theaperture 170. The interior walls of the enclosure preferably have ablack finish, or are baffled as necessary to reduce stray light.

To increase the utilization of light energy, and as shown in FIG. 5, alens 178 is placed between the reflector 150 and the web 142 to increasethe amount of light focused at the focus 162 on the web 142. Theillumination directly from the illuminator 146 at or about 45° towardthe web 142 typically spreads and covers a wide swath on the web 142.The lens 178 is placed such that the lens focus and the focus 162 aregenerally coincident. The lens 178 focuses the direct illumination intothe same line as the elliptical reflected light. The size and placementof the lens are also chosen such that there is no interference betweenthe lens 178 and the reflected light paths.

A circular reflector 182 centered at the first focus 158 is positionedat the blind spot 174. The illumination proceeds from the illuminator146 to the circular reflector 182. From the circular reflector 182, theillumination is reflected back through the illuminator 146 and furtherto the lens 178, which focuses the illumination on the web 142.

If the distance between the circular reflector 182 and the illuminator146 is approximately the same as the distance between the ellipticalreflector 150 and the illuminator 146, the circular reflector 182 andthe elliptical reflector 150 can be fabricated as a single extrudedassembly. In this way, the blind spot no longer requires darkening. Boththe circular reflector 182 and the elliptical reflector 150 arepreferably mirrors, polished enough in order to reflect nearly all theillumination as gloss, but with bumpy surfaces on a millimeter scalesuch that a filament image is not projected on the web 142.

It may be beneficial for the angle created between the web 142 and thestraight line 166 formed between the foci 158, 162 to be slightlygreater than 45°. As shown in FIG. 6, two light rays 190, 194 emanatefrom a single point on the illuminator 146 onto the web 142 therebydefining two angles 198, 202 between the light rays 190, 194 and the web142. The two rays 190, 194 also impinge a scan line 204 on the web 142at two points 205, 206. The first light ray 190, from the illuminator146 to point 205, is on a plane that is perpendicular to the illuminator146. The first angle 198 is 45°, which is appropriate for the desiredgeometry. The second light ray 194, from the illuminator 146 to point206 away from point 205 of the scan line 204, is not on the planeperpendicular to the illuminator 146. As a result, the second angle 202is shallower than 45°. That is, there is a bias toward the light raysthat impinge the web 142 at shallower angles than the desired 45°.Consequently, to achieve the 45° desired geometry on average, the anglebetween the web 142 and the foci 158, 162 is increased by tilting thelighting elements 144 to allow for angles between the web 142 and theline between the foci 158, 162 to be non-ideal, that is, slightlygreater than 45°.

Another embodiment of the reflectors of scanner assembly 134 is shown inFIG. 11, which also illustrates two reflectors 150 a, 150 b with onereflector 150 a downstream of the other. A reflecting surface of each ofreflectors 150 a, 150 b has a general shape in cross section that is aportion of a parabola. As shown, the cross section of the reflectingsurface of the reflector on the right is a portion of the left half of aparabola that has been rotated 45 degrees clockwise from a line parallelto line 500, which extends perpendicular to the web 142. Similarly, thecross section of the reflector on the left is a portion of the righthalf of a parabola that has been rotated 45 degrees counterclockwisefrom a line parallel to line 500. The respective focal points of theparabolas are denoted 502 a and 502 b. The web 142 is illustrated asmoving in longitudinal direction 143. An illuminator (not shown) ispositioned at each respective focal point 502 a, 502 b. Each illuminatorradially emits light toward the surface of its associated reflector. Inparticular, an appropriate illuminator for this arrangement is a tubeshaped bulb, having a radius of about 5 mm, and about four inches long.The long direction of the bulb is parallel to the lateral direction ofthe web (i.e., the bulb would extend perpendicularly to the planeillustrated in FIG. 11, similar to the arrangement shown in FIG. 3).Each reflector 150 a, 150 b extends substantially parallel to anassociated illuminator. As shown, the light reflected by each reflectoris redirected in a set of parallel rays to the web. The web 142 thenreflects this collimated light and the reflected light travels to imagesensor 145 (not shown in FIG. 11), which records image data indicativeof the printed work on the web.

Another embodiment of the arrangement of parabolic reflectors 150 a, 150b is illustrated in FIG. 12. In this case, the cross section of thereflector on the right (150 a) is a portion of the right half of aparabola that has been rotated 45 degrees clockwise from vertical.Similarly, the cross section of the reflector on the left is a segmentof the left half of a parabola that is rotated 45 degreescounterclockwise from vertical. The arrangement illustrated in FIG. 12takes more space vertically as compared to the arrangement illustratedin FIG. 11.

However, for both arrangements, the light rays directed toward the webare collimated and at the desired angle of 45 degrees, and the rays fromboth reflectors overlap in a region denoted by 504. Such an arrangementis advantageous in that the illumination on the web is relativelyconstant, despite the fact that web weave, i.e., movement of the web upand down, may occur from a highest web position to a lowest webposition. Typical web weave may be on the order of 0.75 inches or so. Asmore fully explained below, the use of parabolic reflectors ofappropriate size and spacing from the illuminator allows the width ofthe collimated light directed to the web to be approximately 15 mm andalso allows for the efficient utilization of light.

Several parabolas 506 a-d, having respective focal lengths of 5, 10, 15,and 20 units, and all having a focal point at (0,0) are illustrated inFIG. 13. Each parabola has the following general formula, where c is thefocal length: ${f(x)} = {{- \frac{x^{2}}{4c}} + c}$

Assuming that a bulb radius of the illuminator 146 is 5 mm, and adesired width of collimated light is 15 mm, one important considerationin the selection of an appropriate portion of a parabola and itsarrangement with respect to the bulb is the utilization of light fromthe illuminator. In other words, only some of the light from theilluminator is emitted in the direction of the reflector, with the restunused. The amount of light utilized can be quantified by looking at theangle of light rays that hit the reflector 150 a, 150 b. The greaterthis utilization angle is, the higher the utilization of light. Inparticular, FIG. 14 illustrates various parabolic curves havingrespective focal lengths (c) of 10, 15, 20, 25 and 30 mm, arranged nearan illuminator 146. The illustrated illuminator 146 has a diameter of 10mm, with the light rays being emitted in a radial direction. Assuming anecessary clearance between each reflector and the illuminator of 5 mm,the parabolic curves in FIG. 14 can be used to determine the utilizationangle in order to achieve a 15 mm wide strip of collimated light. Forexample, for c=10 mm, angle ABC represents the utilization angle of thelight that is collimated into the 15 mm wide strip. Similarly, for c=15mm, angle DBE represents the utilization angle of the light that iscollimated into a 15 mm wide strip. For c=20, 25, and 30 mm, the similarutilization angles are respective angles FBG, HBI, and JBK. Theseutilization angles, in degrees, can be plotted as a function of focallength c, as is shown in FIG. 15 for the curve labelled “starts at x=10”(i.e, the portion of the parabola starts at x=10 mm).

By varying the start position of the portion of the parabola, othercurves relating utilization angle to focal length can be generated. Inparticular, the illustrated curve having a maximum value is for aparabolic portion beginning at x=5 mm, and the middle curve is for x=7.5mm. The graph illustrated in FIG. 15 can thus be used to optimize thearrangement and focal length of the parabolic reflector to achieve ahigh light utilization amount. FIG. 15 illustrates that the highestlight utilization arises when the leftmost edge of the parabola is atx=5 mm, or where the reflector would be touching the side of the bulb.Because some clearance between the illuminator 146 and the reflector 150is practically necessary, a clearance of x=7.5 mm is selected. FIG. 15also illustrates that for the edge of the reflector at x=7.5 mm, theutilization angle is at a maximum for a focal length just greater than 9mm. Because this curve is actually fairly flat over this region, a valueof 10 mm is selected for the focal length for an appropriate parabolicreflector.

A second important consideration in the selection of an appropriateparabolic reflector is to insure that the light intensity varies aslittle as possible in the x-direction, in particular from x=7.5 mm tox=22.5 mm. With uniformity, the intensity will not vary as the web movesup and down. By relating the direction (angle) of the light that leavesthe bulb to the lateral position that the light hits the web when theweb is in the nominal position, and differentiating this with respect toposition, one is able to determine the flux density of light.

Define θ to be the angle between the ray emitted from the bulb and thex-axis. Then (k cos θ, k sin θ) is a point on this ray at a distance kfrom the filament of the bulb. Inserting this into the equation for theparabola results in:${k\quad\sin\quad\theta} = {{{- \frac{1}{4c}}\left( {k\quad\cos\quad\theta} \right)^{2}} + c}$

Writing this in the standard form for a quadratic equation:$0 = {{\left( \frac{\cos^{2}\theta}{{- 4}c} \right)k^{2}} - {\left( {\sin\quad\theta} \right)k} + c}$

Using the quadratic equation:$k = \frac{{\sin\quad\theta} \pm \sqrt{{\sin^{2}\theta} + {4\quad\frac{\cos^{2}\theta}{4c}c}}}{{- 2}\frac{\cos^{2}\theta}{4c}}$

Using cos² θ+sin² θ=1, this can be rewritten:$k = {2c\frac{{\sin\quad\theta} \mp 1}{\cos^{2}\theta}}$

The positive root is:$k = {2c\frac{{\sin\quad\theta} + 1}{\left( {1 + {\sin\quad\theta}} \right)\left( {1 - {\sin\quad\theta}} \right)}}$Or: $k = \frac{2c}{1 - {\sin\quad\theta}}$

Since x=k cos θ, $x = {\frac{2c}{1 - {\sin\quad\theta}}\cos\quad\theta}$

Solving for θ in terms of x:$\theta = {\pm {\cos^{- 1}\left\lbrack \frac{4{cx}}{{4c^{2}} + x^{2}} \right\rbrack}}$

FIG. 16 illustrates a plot of angle θ vs x over the range from x=7.5 mmto x=22.5 mm. The flux density is proportional to the derivative of θwith respect to x. This derivative is:$\frac{\mathbb{d}\theta}{\mathbb{d}x} = {\frac{180}{\pi}\left( \frac{4c}{{4c^{2}} + x^{2}} \right)}$

This function is plotted in FIG. 17. The light intensity from a singleilluminator (with a parabolic reflector having c=10 mm, extending from7.5 mm to 22.5 mm) will drop, at x=22.5, to about 60% of the maximumintensity. When the two illuminators are combined, the variation in thetwo lights will unfortunately not cancel each other out. Using thearrangement illustrated in FIG. 1, the light intensity when the web isat the highest position is only 60% of the light intensity when the webis at the lowest position. Using the arrangement illustrated in FIG. 12,the variation is also 60%, but the maximum intensity occurs when the webis at its highest position, rather than the lowest. This suggests acombined approach, as illustrated in FIG. 18, with one reflector at eachorientation, such that the combined light is roughly constant withrespect to height.

FIG. 19 plots the flux density versus distance for the arrangementillustrated in FIG. 18. The flux density ranges from a maximum of 7.55degrees/mm at the ends to 7.33 degrees/mm in the middle. This is roughlya 3% variation, which is a definite improvement compared to the fluxdensity variation for the arrangements of reflectors shown in FIGS. 11and 12.

FIG. 20 illustrates a further improvement to a reflector for anillumination system. Illustrated is a spiral type reflector 510 thatincorporates a parabolic reflecting surface 512 as well as a circularreflecting surface 514. A perspective view of this reflector isillustrated in FIG. 21. The circular reflecting surface 514 is designedso that the filament of the bulb is at the center 516 of the circle, sothat the light and heat exiting the bulb will be directed directly backto the filament. The center 516 of the circle also coincides with thefocal point of the parabola defining the parabolic reflecting surface512. The reflection from the circular reflecting surface 514 serves toincrease the temperature of the filament so that ultimately less poweris needed to achieve the same temperature (light output). This reflectorcan be fabricated from a single piece of metal.

Turning to FIG. 7, the scanner assembly 134 preferably includes aplurality of image sensors 145 such as linescan cameras. Each imagesensor 145 generally covers a specific scan area on the web 142. Theimage sensors 145 are generally arranged laterally across the web 142.The number of image sensors 145 is generally application dependent. Forexample, a single image sensor 145 may adequately cover the web 142 inone application, but more than one image sensor 145 may be required tospan the web 142 in another. In an application where a plurality ofimage sensors 145 is required, partial overlapping of the scan areas maybe necessary to ensure complete web coverage.

Each image sensor 145 preferably includes a plurality of independentimage channels. In one embodiment, there are three channels responsivegenerally to the wavelength ranges 400 to 500 nanometers, 500 to 600nanometers, and 600 to 700 nanometers. These three channels are referredto as the blue, green and red channels, respectively. If thedensitometric fidelity is more important than the colorimetric fidelityin the print work, the spectral responsivity of the three channels willbe designed to comply with the definitions of Status T or Status E asdefined in ISO 5-3, or with the German standard DIN 16536, for example.

If the colorimetric fidelity is more important than the densitometricfidelity, the three channels would be designed to meet the Luther-Ivescondition. Spectral responsivities that meet the Luther-Ives conditionare 1) spectral responsivities that are each a linear combination of thetristimulus functions, as defined in ISO 15-2, and 2) spectralresponsivities that span the three-space of the tristimulus functions.

If no adequate compromise between densitometric fidelity andcalorimetric fidelity can be found with three channels, a set of morethan three channels may be necessary.

With respect to spatial resolution, requirements are typicallyapplication dependent. Applications requiring a high quality ofinspection typically require extremely fine resolution. Applicationsrequiring only detection of image defects that are readily apparent to aobserver do not require extremely fine pixel resolution. In thepreferred embodiment, an image pixel resolution of 75 DPI is chosen forexample. A resolution of 75 DPI is sufficient to detect defects that arereadily apparent to the human eye at arms length, and it is also aresolution sufficiently coarse that halftone screens typically used oncommercial print product will not be imaged as moiréê patterns.

If the requirements for the defect detection subsystem and the colorcontrol subsystem are sufficiently different, or an image sensor withhigher resolution is preferred for reasons of availability or cost, itis possible to re-sample an image to a different resolution for one orboth of the subsystems. Specifically, a full resolution image is firstblurred in a manner consistent with the amount of size reduction, andthe image is subsequently decimated to produce a down-sampled image.Decimation is a process in which a set of data sampled at an originalsampling rate is down-sampled at a lower sampling rate thereby producinga down-sampled set of data. The decimation process occasionallyintroduces staircase-like aberrations on sharp slanted lines. Increasedsmoothing or combining decimation with bilinear interpolation or anyother suitable interpolation procedure typically reduces the staircaseeffect. Since decimation can be performed without applying the initialblurring process to all pixels, both decimation and blurring arecombined to form a more efficient operation.

A flowchart 300 according to the present apparatus and method is shownin FIG. 8. The steps set forth in FIG. 8 are modular in nature anddetail one embodiment of the invention. The operation generally includesfive processes: templating, acquisition, color control, defect detectionand integration. Depending on the application, the operations preferablyrun on the processor 138, such as a conventional general purposecomputer, but can be adjusted to run completely or partially on adigital signal processor, an application specific integrated circuit,specialized digital hardware, pipelined array processors, systolicprocessors, or the like.

Specifically, FIG. 8 includes a templating subsystem module 304, anacquisition subsystem module 308, a color control subsystem module 312,a defect detection subsystem module 316, and a integration subsystemmodule 320. Briefly, in the templating process, a preferably digitalrepresentation is initially created of what should ideally be printed onthe web. This so-called template image is created based on a prepresssource of information. The template image could be created from the datafiles used to create the printing plate, or it could be based on a scanof a proof, for example. When the printed work on the press is ofacceptable quality, an acquired image may also be used as the templateimage. The acquisition process encompasses the collection of an image ofa complete repeat of the print, as well as additional processing tobring this image to a standardized form. The color control process,which is preferably a markless system, entails comparison of thecurrently acquired image against the template image. Based on thiscomparison, recommendations are made for adjustments of inking levels.These recommendations may be fed to an operator, directly to an inkinglevel actuator, or to an external process which is controlling inkinglevels via a PID loop, an adaptive control loop, or to some model-basedcontrol system, for example. The defect detection process entailscomparison of the acquired image against the template image. The purposeof defect detection is to find print defects rather than to adjustinking levels. Therefore, the processing for defect detection after thecomparison will differ substantially from the processing after colorcontrol. The integration process receives inputs from the color controlsubsystem and the defect detection subsystem. Based on these inputs, theintegration process may choose to enable or disable the action of eitherthe color control subsystem or the defect detection subsystem, orperhaps choose to modify any of the outputs.

In normal operation, the templating process will be the first to occur.This will preferably occur in a computer located off-press, andnetworked to various printing presses throughout a plant. During theinitial makeready impressions, the ink levels will be stabilizing andthe inks will be substantially out of register. The integrationsubsystem module will most likely be informed that a substantial amountof defects have been found as compared to the template image, and thatthe color control subsystem does not believe that it can adequatelycorrect the color yet. Based on this, the outputs from the defectdetection subsystem and from the color control subsystem will bedisabled.

Eventually, the inks will all be at some nominal level and registrationwill be reasonable. At this point, the defect detection subsystem willstill see a substantial amount of defects, but the color control systemwill deem the color substantially correctable. Based on this, theintegration subsystem will enable the output of the color controlsubsystem, but will continue to disable the defect detection subsystemoutput. The color control subsystem will work to adjust the inkinglevels on the web to within target tolerances of the colors in thetemplate image. As this happens, the amount of defects detected will bereduced, and the degree of color match will improve.

When the amount of defects and the degree of color match are within aspecified tolerance, the integration subsystem module will enable theoutput of the defect detector subsystem. At this point, the defectdetector subsystem will apprise the operator of any defects that havebeen detected. This may take the form of, for example, an image displaywith an overlay highlighting the places on the web where appreciabledifferences occur. These highlighted defects may be used to diagnose theneed for further adjustment of color, or may indicate a plate scratch orcomposition error. These highlighted defects may also indicateinaccuracies in the process by which the appearance of the web isestimated from the prepress information. Therefore, when the press hasreached the “color ok” stage, it may be desirable to obtain a morerepresentative image of the print on the web by capturing an imagedirectly from the web. At this point, the operator may choose to replacethe template image with an image collected from the web. It is possibleto reduce operating tolerances at this time in either of the colorcontrol subsystem or the defect detection subsystem.

Turning now to the specifics of each module, in the templating subsystemmodule 304, a prepress image 324 is first derived from a digital datafile 328 that is used to image a printing plate. Some applications mayrequire an entire repeat be stored in the image 324, while otherapplications may require only critical portions of the repeat be stored.However, when a template image is created from an online image, it maybe preferable to store multiple repeats as the template image.Alternatively, the prepress image 324 can also be obtained by scanning acontract proof. Using a contract proof to generate the prepress image324 is preferred because defects introduced after the proofing stage maybe flagged by the defect detection system 316. In addition, the contractproof also has an appearance agreed upon by the printer and the printbuyer. Contract proofs typically cover only a single page of amulti-paged repeat. As a result, multiple contract proofs are joinedtogether in mosaic fashion to create an image of the full repeat.

The prepress image 324 format does not always match with that of thescanner assembly 134. Specifically, the pixel size of the prepress image324 does not usually match the pixel size of the image sensors used inthe scanner assembly 134. Therefore, it is generally necessary toresample the prepress image 324 to a pixel size equivalent to the pixelsize of the scanner assembly 134 such as in step 332. Alternately, boththe prepress image 324 and an acquired image are converted to a lowerresolution in order to reduce the computational overhead and memoryrequirements.

The prepress image 324 and the acquired image may not be in the samecolor space, and preferably a color space that exhibits a degree ofperceptual uniformity, such as CIELAB, is utilized. For example, theprepress image 324 may be in CMYK format, whereas the acquired image maybe in RGB format. Thus, it is generally necessary to convert the imagesto a common color space as in step 336. Given the prepress image 324 asan input, the conversion step 336 effectively determines a press imageestimate, that is what the press will produce. A template image 340 isthus obtained, and subsequently stored in template storage 344.

In the acquisition subsystem module 308, images of the web 142 arecontinuously acquired in step 348, such that an image of every line ofevery repeat is collected using a line scanner. If the defect detectionrequirements are stringent, scanning of every portion of the web 142 maybe necessary. The acquisition of an individual line may be triggered bypulses from an encoder coupled with the printing press, for example. Asimages of new lines are being collected, the previously collected linesare processed. The processing includes a correction step 352 fordistortions inherent to the image sensor 145 on a line-by-line basis asthe lines are collected.

The correction step 352 includes a photometric zero subtraction in whicha baseline value indicating an absence of light is subtracted from allthe pixels in a line. However, the baseline value generally varies overtime due to temperature fluctuations, for example. Updated photometriczeros can be obtained from periodically sampling the line scanner withthe illumination disabled, and with the ambient light adequatelyisolated. Step 352 also corrects geometric distortion, such as thegeometric distortion associated with some lens designs. To correct thegeometric distortion, for each pixel in the geometrically correctedoutput line, the graph or formula from the lens design, or the lensempirical measurements can be used to determine the location to retrievethe pixel from the input line. The retrieved location is generally notan integer. Linear interpolation is used to approximate the value to bestored in the geometrically corrected line.

The imaging system as a unit will not typically respond uniformly in allthe pixels. This is due to at least three effects. First, the intensityof the illumination may not be completely uniform. Second, due tovignetting, the lens will capture a wider angle of light from the centerof the field of view. Third, the sensor itself may not be equallyefficient at capturing light in all pixels due to manufacturingimperfections. To correct for such inconsistencies, the image of a lineis divided by a correction line collected from a uniform white object.Other types of image that may require corrections include, but notlimited to, the effects of nonlinear digitization and of scatteredlight, for example.

Colorimetric values, such as CIELAB, are used in the preferredembodiment. The conversion from the regular RGB value to the color spaceor colorimetric values is performed in step 364. In the preferredembodiment, a 9×3 matrix transform is used: $\begin{matrix}{{\begin{bmatrix}X \\Y \\Z\end{bmatrix} = {\begin{bmatrix}0.868 & 0.046 & 0.115 & 0.042 & 0.074 & 0.084 & {- 0.136} & 0.018 & {- 0.037} \\0.425 & 0.527 & {- 0.012} & {- 0.059} & {- 0.031} & 0.031 & 0.174 & {- 0.014} & {- 0.038} \\{- 0.017} & 0.064 & 0.976 & 0.031 & {- 0.003} & 0.000 & {- 0.039} & {- 0.054} & 0.039\end{bmatrix}\begin{bmatrix}R \\G \\B \\R^{2} \\G^{2} \\B^{2} \\{RG} \\{RB} \\{GB}\end{bmatrix}}},} & ({E1})\end{matrix}$where X, Y, and Z, are the standard precursors to the calculation ofCIELAB values.

The translation from RGB values to colorimetric values can be performedin a variety of ways. The coefficients of the transform matrix depend onthe specifics of the spectral response of the scanner assembly 134 andthe illumination used, as well as the reflectance spectra of the inksprinted on the web 142. The transform itself may take any number offorms.

Once step 352 is completed, most of the distortions contributing to thedissimilarities between the acquired image and the prepress image 324have been corrected. What is not known is the precise registration ofthe acquired image relative to the prepress image 324. In order tocompare the acquired image with the template image 340 in subsequentsteps, the two images are aligned in step 356. Specifically, alignmentmay require buffering from a plurality of lines to potentially all thelines of an entire repeat. A number of buffered lines is preferablystored in a memory. Once a predetermined number of lines from roughlythe appropriate area of the image has been stored in the buffer,alignment step 356 takes place.

Alignment of the acquired image to the template image 340 can beperformed in a variety of ways well known in the art. For example,fiducial marks can be printed on the web 142 and located. Alternately,alignment without fiducial marks may also be used. The alignmentfrequency is generally dependent upon how accurate the encoder ticksreflects the actual flow of the web 142. In the preferred embodiment,alignment will be performed once per repeat, although it could beperformed multiple times per repeat, or only once per multiple repeats.Note that if the lateral stretch of the web 142 has sufficientvariability compared to the pixel size of the scanner assembly 134, itmay be necessary to also perform alignment in sections across the web142.

After the alignment step 356 has been completed, correction for anotherdistortion of the scanner assembly 134 is preferably performed in step360. Normal fluctuations in the intensity of the illumination of the web142 will cause an otherwise ideal acquired image to have a differentbrightness and chronia with respect to the template image 340. Step 360corrects the illumination intensity by first averaging the intensitiesof a plurality of preselected areas on the acquired image. Correspondingareas of the prepress image 324 are also averaged. The entire acquiredimage is subsequently scaled such that the template image average andthe acquired image average are the same. Depending on light sourcestability and the web speed, the normalization process in step 360 maybe performed on a line-by-line basis or on a multi-line basis, butpreferably on a repeat-by-repeat basis. Furthermore, the pre-selectedareas may be user defined or set up to include all the pixels in asingle line, multi-line section, or repeat, whether the pixels are inkedor non-inked, for example. The pre-selected areas are preferably thenon-inked portions of the web 142. Automatic identification of theseareas could be based on the prepress information and a sensitivitymatrix defined hereinafter.

Once the calorimetric values have been normalized for illumination instep 360, the data is sent to a comparison step 368 which generatesresults that are shared by both the color control subsystem module 312and the defect detection subsystem module 316. In step 368, thecorrected and color converted acquired image is subtracted from thetemplate image 340.

Referring now to the defect detection subsystem module 316, the processof defect detection begins with the subtraction of the corrected andcolor converted online image from the template image in step 368. Adefect in a pixel is detected in step 376 when a difference between thepixel value on the acquired image and the pixel value on the templateimage 340 is outside a pre-specified threshold. The threshold may bespecified as an absolute difference of either L*, a* or b* that isgreater than a predetermined number, for example, 5. Alternatively, thethreshold may be specified as a AE value that is greater than a secondpredetermined number, for example, 10. In the preferred embodiment, aCMC color differencing formula is used, with a threshold valuedetermined by the quality requirements of a print job and an ability ofthe press to maintain the color.

The presence and (x, y) locations of these potential defects may be allthat is required for some applications. In this case, the connectivityanalysis step 380 will be minimal. The presence or absence of a defectmay be used to trigger a mechanism by which the corresponding impressionmay be marked as defective, or shunted into a different workflow fromthe non-defective product after the web 142 has been cut into individualsignatures. The defect locations may be logged to a data file forstatistical process control purposes. Alternately, an acquired imagewith the defect area highlighted may be displayed to a pressman.

In other applications, further discrimination of defects may berequired. In particular, the size or intensity of the defects may be ofimportance. The size of a defect may be determined by defect orconnectivity analysis in step 380. The result of the thresholding instep 376 may be considered as a binary defect image, with a “1” in apixel indicating a defective pixel, and a “0” in the pixel indicatingotherwise. In the connectivity analysis step 380, adjacent defectivepixels are joined into a single defect particle. The information in thebinary image will thus be reduced to a list of defect particles, eachwith a plurality of defective pixels.

If it is desired that only defects above a predetermined size bereported, a binary morphological operation such as binary erosion may beused in step 380. The original binary defect image is eroded so that alldefects are reduced in size, and only defects that are larger than asingle pixel remain. The erosion process may be repeated to erode moreof the eroded binary image. Each erosion removes the outer rim of pixelsfrom a defect. If it is desired, for example, that only defects with aradius greater than six pixels be reported, erosion has to be performedsix times. At the end of the erosion processes, pixels having a “1”indicate a defect which is larger than the predetermined size. It maythen be desirable to refer back to the original binary defect image tolocate all the pixels associated with the defect.

The defect locations reported by the defect detection subsystem 316 maybe used to decide which pixels are used by the color control subsystem312. To this end, the color differences computed in step 368 are sent toa pixel selection step 370. The pixel selection step 370 passes onlythose pixels that have been selected by a combination of the pressoperator, the original customer of the printed work, and some automatedanalysis program. Alternately, the pixel selection step 370 may make useof only the pixels in the colorbar such as in a marked color controlsystem. The computational load for the color control subsystem may thusbe reduced. Additionally, the pixel selection step 370 may suppress suchpixels that are deemed defective in step 380.

The color differences are then used to determine the color error in thecolor control subsystem module 312 which attempts to minimize the colorerror by adjusting a set of ink metering devices in step 372. The errorminimization process first assumes that for small changes in inkmetering, the relationships in equations E2, E3 and E4 are reasonableapproximations to the actual relationships between the variablestherein. $\begin{matrix}{{L_{p}\left( {x,\quad y} \right)} = {{L_{0}\left( {x,\quad y} \right)} + {\sum\limits_{i}\quad{{k_{\Delta}\left( {i,\quad j} \right)}{F\left( {x,\quad y,\quad j} \right)}{S_{L}\left( {x,\quad y,\quad i} \right)}}}}} & \left( {E\quad 2} \right) \\{{a_{p}\left( {x,\quad y} \right)} = {{a_{0}\left( {x,\quad y} \right)} + {\sum\limits_{i}\quad{{k_{\Delta}\left( {i,\quad j} \right)}{F\left( {x,\quad y,\quad j} \right)}{S_{a}\left( {x,\quad y,\quad i} \right)}}}}} & \left( {E3} \right) \\{{b_{p}\left( {x,\quad y} \right)} = {{b_{0}\left( {x,\quad y} \right)} + {\sum\limits_{i}\quad{{k_{\Delta}\left( {i,\quad j} \right)}{F\left( {x,\quad y,\quad j} \right)}{S_{b}\left( {x,\quad y,\quad i} \right)}}}}} & \left( {E4} \right)\end{matrix}$where,

-   -   (x, y) represents coordinates of a pixel in the acquired image        or the template image 340,    -   L_(o)(x, y), a_(o)(x, y), and b_(o)(x, y) represent the CIELAB        values of the online image at location (x,y),    -   k_(Δ)(i, j) represents a change in the amount of ink number i        (for example, with i=1 being cyan, i=2 being magenta) metered at        lateral position j, where j goes from 1 up to the number of ink        metering devices across the web 142,    -   F(x, y, j) represents the relative effect that ink metering        device j has on pixel (x,y),    -   S_(L)(x, y, i), Sa (x, Y, i), and S_(b)(x, y, i) are three        dimensional sensitivity matrices that estimate the amount of        change there will be in L*, a*, and b*, respectively, at a point        (x, y) for a unit change in kA (i, j), and    -   L_(p)(x, y), a_(p)(x, y), and b_(p)(x, y) represent the        predicted CIELAB values of the acquired image at location (x,        y), after a change in the ink metering as specified by the k_(Δ)        vector.

Due to the spread of ink by the vibrator rollers, an ink metering devicewill typically provide ink to a somewhat wider area than the actualwidth of the ink metering device. As a result, if information of the inkspread is available during the make-ready process, the convergence timecan be improved especially when the ink metering devices require largechanges. For example, one value for F(x, y, j) is 0.5 for pixels withinthe width of the ink key metering device, and another value is 0.2 forthe pixels in the neighboring areas. The value of F(x, y, j) can bechanged at color ok to reflect no ink spread.

Equations E2, E3 and E4 are a linear set of equations in k_(Δ)(i, j). Todetermine the required changes in ink metering in step 372, a residualerror as shown in Equation E5 is first set up: $\begin{matrix}{\varepsilon = {\sum\limits_{x,\quad y}\quad\sqrt{\left\lbrack {\left( {{L_{p}\left( {x,\quad y} \right)} - {L_{t}\left( {x,\quad y} \right)}} \right)^{2} + \left( {{a_{p}\left( {x,\quad y} \right)} - {a_{t}\left( {x,\quad y} \right)}} \right)^{2} + \left( {{b_{p}\left( {x,\quad y} \right)} - {b_{t}\left( {x,\quad y} \right)}} \right)^{2}} \right\rbrack}}} & ({E5})\end{matrix}$where L, (x, y), a_(t)(x, y), and b_(t)(x, y) represent the CIELABvalues of the template image 340 at location (x, y). The quantity beingsummed is the standard color difference between corresponding pixels.The required ink changes are determined by obtaining a vector k_(Δ)(i,j) that minimizes the residual error,

. Alternatively, the changes can be determined from a differencingformula such as the CMC color differencing formula.

This is an overdetermined linear system. It is therefore possible to usestandard regression techniques to determine the minimization.

In the preferred embodiment, images will be taken of every impression.In a typical web offset printing press, a change in the ink metering maytake hundreds of impressions to be fully expressed. AProportional-Integral-Derivative (“PID”) loop could be tuned to dealwith the long delay. The color control subsystem module 312 willpreferably wait for a number of impressions after issuing a change inink metering before requesting a subsequent change. In this way, thecomputational load on the system is decreased.

The sensitivity matrices, S_(L)(x, y, i), S_(a)(x, y, i), and S_(b)(x,y, i), may be estimated by analyzing the effect of changes in inkinglevels. In one embodiment, estimates about the ink composition atvarious points in the impression may also be made based on knowledge ofthe typical color values for various combinations of inks.

Turning now to the integration subsystem module 320, this module enablesor disables the inking control or the defect outputs from the colorcontrol subsystem module 312 and the defect detection subsystem module316, respectively, depending on the outputs of the modules 312, 316. Theinformation from these two modules 312, 316 determines the state of theprinting press and also the appropriateness of the enabling anddisabling outputs. For example, the defect detection subsystem ispreferably disabled if it is determined that the defects found arelargely the result of the color being incorrect. An estimate of the timethat it will take to correct the color as well as the magnitude of thedefects may be used as a basis for disabling the defect detectionsubsystem. Further, by determining when color is within a giventolerance, it is possible to tighten the defect tolerance sincespuriously detected color defects would be eliminated.

The information received by the integration subsystem module 320 fromthe color control subsystem module 312 may include the residual colorerror, ε determined from equation E5. The value of ε indicates how closethe template image 340 and the acquired image will be once the requestedinking change has stabilized on press.

In addition, the information received from the defect detectionsubsystem module 316 may include the sum of defects, δ. The sum ofdefects, δ indicates how close the template image 340 and the currentacquired image are: $\begin{matrix}{\delta = {\sum\limits_{x,\quad y}\quad\sqrt{\left\lbrack {\left( {{L_{0}\left( {x,\quad y} \right)} - {L_{t}\left( {x,\quad y} \right)}} \right)^{2} + \left( {{a_{0}\left( {x,\quad y} \right)} - {a_{t}\left( {x,\quad y} \right)}} \right)^{2} + \left( {{b_{0}\left( {x,\quad y} \right)} - {b_{t}\left( {x,\quad y} \right)}} \right)^{2}} \right\rbrack}}} & ({E6})\end{matrix}$

Note that if k_(Δ)=0 in equations E2, E3 and E4, L_(o)(x, y)=L_(p)(x,y), a_(o)(x, y)=a_(t)(x, y), and b_(o)(x, y)=b_(p)(x, y), and hence,ε=δ. Since ε is determined from a minimization process, it follows thatε≦δ will always be the case.

One possible set of rules, for example, for the output control is shownas Table 384 in FIG. 9. Table 384 uses ε and δ, as defined in equationsE5 and E6, as inputs respectively. Table 384 also uses “Previousprediction,” which indicates a previous value of the residual colorerror, ε, with time scale taken such that any color changes would havestabilized. If the color control was to be disabled at any step, thenext value for “Previous prediction” would preferably be set to thecurrent value of the residual color error ε.

The rules set may be modified to include more than two values such as,for example, “Small,” “Medium,” and “Large.” The rules may also includea larger number of previous states. Implementation can be based on astate machine, a neural network, or fuzzy logic. Similarly, the rulesmay be laid out explicitly as a series of “if-then” statements.

The computations of ε and δ, and the application of the rules may beapplied based on a full impression. As a result, the enabling ordisabling the color control output or the defect detection output isbased on the entire impression. Alternately, the enabling and thedisabling action may be applied separately to individual alleys, or inkkey zones, as required by the application.

Furthermore, the defect detection subsystem 316 also operates to keepthe color control subsystem 312 from making decisions simply based ondefective pixels. For example, the color control subsystem 312 will bedisabled in the event of a blanket wash, or other such severe defectsuch that only few inked pixels are detected. The integration module 320may also elect to disable inking control outputs based on whether thecompute ink key adjustments module 372 has an adequate pixel count orratio of allowed pixels to possible pixels. Alternately, the colorcontrol subsystem 312 may also be disabled based on a numerical analysison the stability of the solution of the linear equations representingthe system, or a condition number or a singular value decomposition ofthe relevant matrices of the system. Other severe condition that maydisable the color control subsystem 312 includes a startup condition ofthe printing press. Specifically, the inking levels may be substantiallyoff during the startup of the printing press. When the inking levels aresubstantially off, the defect detection subsystem 316 will label a largequantity of pixels defective thereby undesirably disabling the colorcontrol subsystem 312.

As shown in FIG. 8, the pixel selection module 370 limits the number ofpixels that are suppressed to avoid undesirable disabling of the colorcontrol subsystem 312. For example, if suppression is required by morethan half of the pixels in an acquired image, the pixel selection module370 then passes along only those pixels with the smallest errors. Inanother embodiment, the output of the defect analysis module 380 is fedinstead to a second compute ink key adjustment module. The secondcompute ink key adjustment module will perform an actual inking control.In this way, the defect analysis module 380 provides information fortrue defect suppressions, but not the defects that cover the entire web142. Furthermore, the initial computation of the original ink keyadjustments in module 372 will be made based on all the pixels, exceptfor those requiring suppression for other reasons.

FIG. 8 also shows a single output from the defect detection module 316.Some applications may include more outputs with different criteria. Forexample, one output may be the data from which visualizations of thedefects are constructed. Another output may indicate whether a givenimpression contains an error sufficiently large to warrant diverting thecorresponding impression from the acceptable print.

Sharing of image acquisition and processing by the color control anddefect detection control systems of the present invention reduces theoverall cost of the control system, reduces maintenance costs, as wellas reduces the space needed to house the control system.

The preferred embodiment uses prepress information in advantageous ways.A prepress representation is first used as a template during makereadyfor both the defect detection and the color control in step 328. Thesensitivity matrices are also computed from the prepress information instep 334. Furthermore, areas where there is no ink coverage aredetermined by analyzing the prepress information in module 316. This, inturn, is used to select pixels to be used for normalization ofillumination levels.

In the absence of the prepress information, an alternative embodimentthat does not require the prepress information can be used. For example,the acquired image corrected in the acquisition module 308 can be usedas a template. During makeready, the defect detection subsystem module316 will generally not be used, and the color control subsystem module312 may be either disabled or based solely upon color patches within acolor bar. Therefore, there will be enough time for an adequate acquiredimage to be acquired and stored as a template image 340.

FIG. 10 illustrates an alternative embodiment of a control system 400according to the present invention. A printed web 404 moves passes adefect detection system scanner 408 in a direction indicated by arrow412. The defect detection system scanner 408 contains an array oflighting elements, such as those described earlier, and an array ofimage sensors. The lighting elements and the image sensors are generallyarranged laterally across the scanner 408 and perpendicular to thedirection of the moving web 412. Depending on the application, thescanner 408, the lighting elements, and the image receptors can bearranged differently.

The defect detection system scanner 408 scans to acquire image datarepresentative of the printed web 404. The scanned image data issubsequently transferred to a defect detection system processor 416 forfurther processing including a comparison of the acquired image with atemplate image stored in the processor 416. All the discrepanciesbetween the template image and the acquired image that are outside ofsome predetermined threshold or tolerance are considered as defects, andlocations at which defects are detected are also identified. The defectdetection system processor 416 then transfers the defect locations to acolor control system processor 420.

After the web 404 has moved past the defect detection system scanner408, the web continues to move in the same direction 412. As the web 404moves below the color control system scanner 424, the color controlsystem scanner 424 acquires image that is representative of the printedweb 404. Similar to the defect detection system scanner 408, the colorcontrol system scanner 424 typically contains an array of lightingelements and an array of image receptors.

The color control system scanner 424 passes the image data to the colorcontrol system processor 420 for further processing. Typical processingincludes color value conversion which converts the image data into itscorresponding color values for an individual pixel or a group of pixels.Other processing includes assembling the image data into a plurality oflines and aligning the lines with a color control image template.

Furthermore, if the defect detection system processor 416 detects nodefect with a predetermined number of lines, the color control systemprocessor 420 performs only a comparison between the color values andthe color control image template. When a difference is detected by thecolor control system processor 420, changes in inking level aregenerated and sent to a press interface.

It should be noted that preferably, the color control subsystem 312 ofthe present invention is of the markless color control type. However,the invention can be utilized with conventional color patch colorcontrol. Furthermore, depending on application, the present inventionallows for ink key zone control and monitoring as well as the controland monitoring of the whole web.

Various features and advantages of the invention are set forth in thefollowing claims.

1. An illumination system for a web travelling from upstream todownstream in a longitudinal direction in a printing press, with alateral direction substantially perpendicular to the longitudinaldirection, the system comprising: a first and a second illuminator foremitting light, each illuminator having a long axis arranged in thelateral direction, and a first and a second reflector, the secondreflector arranged downstream from the first reflector, each reflectorhaving a surface for reflecting light from a corresponding illuminatortoward the web, wherein a cross-section of each reflecting surface is aportion of a parabola having a focal point, and wherein a correspondingilluminator is centered at each focal point.
 2. The illumination systemof claim 1, wherein light rays from each reflector hit the web at anangle of substantially 45 degrees from the plane of the web.
 3. Theillumination system of claim 1, wherein the first and second reflectorsare symmetric to each other about a plane perpendicular to the web. 4.The illumination system of claim 3, wherein the reflecting surface ofthe first upstream reflector reflects a portion of the light emittedfrom the first illuminator that is directed substantially away from theweb, and the reflecting surface of the second downstream reflectorreflects a portion of the light emitted from the second illuminator thatis directed substantially away from the web.
 5. The illumination systemof claim 3, wherein the reflecting surface of the first upstreamreflector reflects a portion of the light emitted from the firstilluminator that is directed substantially toward the web, and thereflecting surface of the second downstream reflector reflects a portionof the light emitted from the second illuminator that is directedsubstantially toward the web.
 6. The illumination system of claim 1,wherein the first and second reflectors are not symmetric to each other.7. The illumination system of claim 6, wherein the reflecting surface ofthe first upstream reflector reflects a portion of the light emittedfrom the first illuminator that is directed substantially away from theweb, and the reflecting surface of the second downstream reflectorreflects a portion of the light emitted from the second illuminator thatis directed substantially toward the web.
 8. The illumination system ofclaim 6, wherein the reflecting surface of the first upstream reflectorreflects a portion of the light emitted from the first illuminator thatis directed substantially toward the web, and the reflecting surface ofthe second downstream reflector reflects a portion of the light emittedfrom the second illuminator that is directed substantially away the web.9. An illumination system for a web travelling from upstream todownstream in a longitudinal direction in a printing press, with alateral direction substantially perpendicular to the longitudinaldirection, the system comprising: a first and a second illuminator foremitting light, each illuminator having a long axis arranged in thelateral direction, and a first and a second reflector, the secondreflector arranged downstream from the first reflector, each reflectorhaving a compound reflecting surface for reflecting light from acorresponding illuminator toward the web, wherein a cross-section ofeach compound reflecting surface includes a first curve that is aportion of a parabola having a focal point, and a second curve that is aportion of a circle centered at the focal point, and further wherein acorresponding illuminator is centered at each focal point.
 10. Theillumination system of claim 9, wherein light rays from each reflectorhit the web at an angle of substantially 45 degrees from the plane ofthe web.
 11. The illumination system of claim 9, wherein the first andsecond reflectors are symmetric to each other about a planeperpendicular to the web.
 12. The illumination system of claim 11,wherein the reflecting surface of the first upstream reflector reflectsa portion of the light emitted from the first illuminator that isdirected substantially away from the web, and the reflecting surface ofthe second downstream reflector reflects a portion of the light emittedfrom the second illuminator that is directed substantially away from theweb.
 13. The illumination system of claim 11, wherein the reflectingsurface of the first upstream reflector reflects a portion of the lightemitted from the first illuminator that is directed substantially towardthe web, and the reflecting surface of the second downstream reflectorreflects a portion of the light emitted from the second illuminator thatis directed substantially toward the web.
 14. The illumination system ofclaim 9, wherein the first and second reflectors are not symmetric toeach other.
 15. The illumination system of claim 14, wherein thereflecting surface of the first upstream reflector reflects a portion ofthe light emitted from the first illuminator that is directedsubstantially away from the web, and the reflecting surface of thesecond downstream reflector reflects a portion of the light emitted fromthe second illuminator that is directed substantially toward the web.16. The illumination system of claim 14, wherein the reflecting surfaceof the first upstream reflector reflects a portion of the light emittedfrom the first illuminator that is directed substantially toward theweb, and the reflecting surface of the second downstream reflectorreflects a portion of the light emitted from the second illuminator thatis directed substantially away the web.
 17. An illumination system for aweb travelling from upstream to downstream in a longitudinal directionin a printing press, with a lateral direction substantiallyperpendicular to the longitudinal direction, the system comprising: afirst and a second illuminator for emitting light, each illuminatorhaving a long axis arranged in the lateral direction, and a first and asecond reflector, the second reflector arranged downstream from thefirst reflector, each reflector having a surface for reflecting lightfrom a corresponding illuminator toward the web, wherein a cross-sectionof each reflecting surface is a portion of an ellipse having a first anda second focus, wherein the first illuminator is centered at the firstfocus of the first reflector, and the second illuminator is centered atthe first focus of the second reflector, and the second focus of thefirst reflector and the second focus of the second reflector aresubstantially coincident.
 18. The illumination system of claim 17,wherein a first angle between the web and a line connecting the firstfocus and the second focus of the first reflector is 45 degrees orslightly greater than 45 degrees, and a second angle between the web anda line connecting the first focus and the second focus of the secondreflector is 45 degrees or slightly greater than 45 degrees.
 19. Theillumination system of claim 17, wherein the first reflector and thesecond reflector both include a blind spot.